1. Field of the Invention
The present invention relates to quartz glass used in the semiconductor industries and optical communications and a method for the preparation thereof and, more particularly, relates to quartz glass such as, for example, a quartz glass crucible for pulling-up of a single crystal of silicon containing a very small amount of the element of zirconium as well as a method for the preparation thereof.
2. Description of the Related Art
In the preparation of quartz glass by fusion and vitrification of natural quartz as the starting material, heretofore, preparation is conducted by, when high-purity quartz glass is required, subjecting a starting material to heating and fusion after upgrading the purity through several kinds of unit refining procedures.
The principal unit refining procedures include pulverization, particle size classification, flotation, magnetic separation, etching, high-temperature heating, heating treatment in a halogen-containing atmosphere, electrolytic refinement and the like.
Even by using a starting material imparted with a high purity in this manner and preventing contamination during fusion by heating as far as possible, several elements cannot still be completely removed. And, some of them are the elements which cause degradation of the performance when used in the semiconductor industries or for the optics. The progress of the semiconductor industries and the optical technologies in recent years has come to a stage on which a content of the level heretofore admissible is no longer admissible as a detrimental content.
As to the element of zirconium, in particular, while it is construed to be natural that the content thereof in natural quartz glass is about 1 ppm or so, it is desired to accomplish a further decrease, preferably, not to exceed 0.8 ppm. Incidentally, the unit of ppm in this specification for the expression of the concentration refers to "ppm by weight" giving the concentration in weight.
Even when a high-purity quartz powder is obtained by fully utilizing the conventional refining procedures mentioned above so as to ensure a content of the element of zirconium not to exceed 0.2 ppm by analysis, however, there can be a disadvantage that the quartz glass obtained by the fusion thereof may eventually contain about 1 ppm of the element of zirconium.
When a quartz glass crucible for pulling-up of a single crystal prepared in this way containing about 1 ppm of the content of the zirconium element was used, a high-quality single crystal of silicon as in the use of a synthetic quartz glass crucible of a content of the zirconium element not exceeding 0.2 ppm could not be obtained.
The present invention, which has been completed in view of the above described situations, has an object to provide high-purity quartz glass with high purity and, in particular, containing little zirconium (Zr) prepared at low costs from natural quartz as the starting material as well as a method for the preparation thereof.